1. Field of Invention
The present invention relates to a marking method, and more particularly, to a method for forming a mark on the surface of a CO2 laser-transparent material.
2. Description of the Related Art
At present, laser is a marking technique widely used in the industry, and is applied to materials such as plastic, rubber, ceramics, metal, and silicon wafer. Compared with conventional manners, for example, mechanical engraving, chemical etching, screen printing, and ink printing, laser marking has the advantages of rapid production, high flexibility, and being controllable via a computer system. In addition, a prominent characteristic of laser marking is the permanence of the mark generated by a laser on the surface of a workpiece.
There are many kinds of lasers and the femtosecond laser, excimer laser, or Nd:YAG laser are mostly used in silicon wafer marking. However, these lasers are generally very expensive, and the processing mechanism thereof is ablating the surface of the silicon wafer with a laser beam of high energy, which may damage the surface structure of the silicon wafer and result in many flying minute particles, i.e., the so-called “splashing fragments”. The fragments are prone to be attached to the silicon wafer, thus becoming difficult to erase. When proceeding to the subsequent device circuit process, a grip head is used to fix the edge of the silicon wafer. However, the clamping force is easy to make the residual fragments fall off and cause another splashing, which not only contaminates the process, but also severely affects the yield and quality of the product. Moreover, these lasers remove the surface of the product to form a mark, so the mark cannot be re-made, and once marked incorrectly, the product will be abandoned for uselessness, and the material cannot be recycled.
Moreover, in the conventional fabrication process of semiconductor devices, the marking process is generally performed after the silicon wafer is diced into chips. As the technique is being constantly updated and the integrated circuits are becoming lighter, thinner, and smaller, the processing technique has also evolved into dicing the wafer after marking, so as to improve the efficiency of production and operation. However, as the size of the silicon wafer is getting larger, the thickness thereof stays unchanged or becomes smaller. Therefore, when the surface of the silicon wafer is ablated with a laser beam of high energy, a large amount of stress is easily accumulated on the surface of the silicon wafer, resulting in deformation and warping thereof. Though the stress can be eliminated by high temperature annealing, the basic property of the silicon wafer is greatly affected, which is disadvantageous for the subsequent production.
In view of the disadvantages of using the above lasers, ROC (TW) Patent Publication No. 350797 provides a processing method for removing particles in the semiconductor industry, and particularly for removing silicon particles generated after making a mark with a laser on the chip. In the method, the wetting and catalytic effects are achieved with the hydroxyl in the aqueous ammonia, so as to oxidize the particles. ROC (TW) Patent Publication No. 434749 provides a marking method, in which the wafer mark can be recovered after a chemical-mechanical polishing process is performed on the wafer, and no silicon particles are generated during the marking. According to the method, the photoresist is exposed with a fiber optic cable, so as to form a mark on the photoresist, and a wafer mark is formed subsequently by etching with the photoresist having a mark formed thereon as a mask. ROC (TW) Patent Publication No. 359885 provides a method, in which a mark pattern on a tape is defined with a laser beam, then the tape is adhered onto a silicon wafer, then the pattern is transferred to the wafer by a wet or dry process, and the tape is finally stripped to finish making a mark on the silicon wafer. The above method can avoid causing splashing fragments.
In Japanese Patent Publication No. 11-260675, a spot-shaped mark is fabricated on a silicon wafer with a laser, and a layer of transparent thin film is formed thereon. When a laser beam passes through the transparent thin film to make the spot-shaped mark regionally melt and deformed, a plurality of spot-shaped marks can be formed. This method can prevent the splashing fragments generated during the laser processing from being attached to the silicon wafer, and the definition and visibility are ensured by the shape of these spot-shaped marks.
ROC (TW) Patent Publication No. 1233197 provides a chip scale mark and a marking method of the same. According to the method, when a laser beam ablates the surface of a silicon wafer, the chip size mark is used to stably keep the laser system and the marking distance between the wafers by removing the wafer warp on the wafer support. ROC (TW) Patent Publication No. 200538304 provides a method for making a mark by forming an interference fringe on a body to be marked with a laser beam.
Therefore, it is necessary to provide a method for forming a mark on the surface of a CO2 laser-transparent material e.g. silicon to solve the above problems.